Swirlers are used as mixing devices in various technical applications. Optimization of swirlers aims at reducing the energy required to obtain a specified degree of homogeneity of a mixture. In continuous flow mixing the pressure drop over a mixing device is a measure for the required energy. Further, the time and space required to obtain the specified degree of homogeneity are important parameters for the evaluation of mixing devices or mixing elements. Swirlers are typically used for mixing of two or more continuous fluid streams. Axial swirlers are most commonly used as premixers in gas turbine combustors. A so-called swirl number sn characterizes the swirl strength of an axial swirler. The swirl number is defined as the ratio between the axial flux of azimuthal momentum and the axial flux of axial momentum multiplied by the swirler radius. The swirl number is an indication of the intensity of swirl in the annular flow induced by the swirler.
Swirl burners are devices that, by imparting sufficiently strong swirl to an air flow, lead to the formation of a central reverse flow region (CRZ) due to the vortex breakdown mechanism which can be used for the stabilization of flames in gas turbine combustors.
Targeting best fuel-air premixing and low pressure drop is often a challenge for this kind of devices. Good fuel-air premixing must be in fact achieved in a mixing region before the CRZ where the flame is stabilized. This implies the need in this mixing region of sufficiently high pressure losses, i.e. the use of a swirler with sufficiently high swirl number which allows the tangential shearing necessary to well premix fuel with air. High swirl number flows however give also origin to strong shearing at CRZ with too large and unnecessary pressure losses just in this region.
An improvement to the standard design of axial swirl burner has been proposed in U.S. 2012/0285173. This improvement consists in the introduction of a lobed trailing edge which can create small scale counter-rotating vortices embedded into the main vortex and able to enhance fuel-air mixing without significant effect on the swirl number of the main vortex. This solution, which has its origin in the application of lobes to non-swirling devices (disclosed in EP 2 522 912), allows to achieve improved fuel-air mixing also at low swirl numbers of the main swirling flow, with a benefit on pressure losses at the CRZ.
The use of these existing design concepts (standard and lobed swirlers) carries however several risks and disadvantages. In case of the lobed axial swirler, the main risk is flow separation at the trailing edge due to change in the exit flow angle taking place too late along the chord of the swirler. A second deficiency is given by the formation of rotating secondary flow structures in the swirler vanes which, carrying the fuel around, make rather challenging the control and optimization of fuel spatial distribution (spatial un-mixedness). In addition, the strong distortion along the trailing edge given by the lobed structure represents, on its own, a major manufacturing difficulty.
For all these reasons, there is a need for the new swirlers that could allow reduced pressure drop, robust flashback characteristics and improved NOx (due to better mixing), but also keep design relatively simple.